One of the important operating parameters of refrigeration or air conditioning systems using a compressor is the compression ratio, i.e. the ratio of the pressures between upstream and downstream of the compressor, between intake and discharge.
This ratio, which will be designated in the course of this text by the letter "r" defines indeed one of the major thermodynamic characteristics of the cycle, governing in particular the thermodynamic efficiency.
Especially in rotating volumetric compressors, such as vane or screw compressors, normally associated with a fixed built-in compression ratio, the efficiency decreases rapidly when the system compression ratio differs significantly from the built-in one.
By "built-in compression ratio", we mean the pressure ratio determined by the geometry of the compressor itself. By "system compression ratio", we mean the ratio between the exhaust pressure and the intake pressure. These ratios are generally different, the system compression ratio depending on what is connected to intake and to the exhaust of the compressor. As an explanatory example, if the intake and the exhaust would be both connected to the atmosphere, the system ratio would be nearly equal to one.
Thus, different devices have been conceived, such, for instance, as those disclosed in U.S. Pat. No. 3,088,659, to allow varying at leisure the built-in compression ratio so as to make it come near the ratio encountered during operation.
A simple case of implementation consists in providing a machine not with an infinity of compression ratios, which requires complex and costly devices such as shown, for example, in FRG application No. 3,143,193 or in U.S. Pat. No. 4,362,472, but only two ratios.
The implementation is indeed very easy as for instance a simple piston, capable of assuming two positions, actuates the compression ratio control slide visible on FIG. 1 of U.S. Pat. No. 3,088,659; each position corresponds to one of both selectable built-in compression ratios. This arrangement, though not as perfect as a device providing an infinity of compression ratios, results in efficiencies within 2% of the ideal solution with a range of compression ratios between 2,5 and 6, whereas the losses can reach 8 to 10% when the compressor relies upon only one built-in compression ratio.
The problem is then to provide a measuring device allowing to "read" the system compression ratio and, from this reading, to give the necessary instructions, i.e. for example to set or to cancel the pressure on the piston actuating the built-in ratio adjusting means.
Various devices have been conceived or may be conceived to perform this measurement, e.g. the use of two pressure sensors the outputs of which, converted into electric values, are compared and transformed into an electric signal actuating a solenoid valve.
But such devices are complex, due to the need of transforming the pressures into electric signals and of re-converting the electric instruction back into a pressure signal. On the other hand, the conventional piston or membrane devices easily measure a difference between pressures, but not a ratio.